THE FERN 135 



1. In cross sections of the stems or leafstalks note the 

 fibro-vascular bundles and regions of thick- walled rigid 

 tissue (sclerenchyma) differentiated from the thin-walled 

 fundamental or ground tissue (parenchyma). Show their 

 distribution in an outline drawing. The outer layer of 

 cells will be clearly differentiated as an epidermis if the 

 structure studied is aerial, — that is, above ground. Why ? 

 What is the chief purpose of the rigid tissue ? What are 

 some of the functions of the fibro-vascular bundles ? 



2. In cross sections of fibro-vascular bundles, under h.p., observe 



(1) large, thick-walled, empty cells, called tracheids; these with 

 small, thick-walled cells in the interior constitute the wood, or xylem ; 



(2) the surrounding soft, thin-walled tissue, called bast, or phlo'em, 

 containing sieve tubes (see Principles, Fig. 274) ; (3) the bundle sheath 

 and phloem sheath, two adjacent cell membranes inclosing the bast 

 and wood. 



Study these elements of the bundle in stained preparations (Sec. 

 212) of cross and lengthwise sections, making detailed drawings. 



D. The cell structure, or histology, of the frond. 



1. Strip off the epidermis from the lower surface of the 

 frond and mount in water, with the outer surface of the 

 epidermis uppermost on the slide. Note the pores, or sto- 

 mata, with their guard cells, and draw these with adjacent 

 epidermal cells. 



2. Strip off epidermis from the upper surface. Has it 

 stomata ? 



3. Sections of the blade will show the green parenchyma, 

 mesophyll, and air spaces within the leaf opposite the 

 stomata. The cells of the green tissue on the upper side 

 of the frond are usually arranged' in a firm layer, or pali- 

 sade. Note that the veins are fibro-vascular bundles giving 

 strength to the expanded blade, besides carrying water to 

 all parts of it. 



E. The structure of the root tip. Study lengthwise microtome sections of 

 the root tip (Sec. 212). Note the root cap over the growing point, which, 

 consists of a pyramidal apical cell whose apex points inwards. Segments 



